1. Field of the Invention
This invention relates to a process for the production of a high molecular weight film and more particularly, it is concerned with a process for the production of a polyethylene terephthalate film having improved dynamic properties.
2. Description of the Prior Art
As is well known by the disclosure of U.S. Pat. No. 2,823,421, polyethylene terephthalate (which will hereinafter be referred to as PET) is capable of being formed into a film having excellent dynamic, optical and chemical properties by subjecting an amorphous and non-orientated sheet obtained by melt extrusion to elongation in two directions, vertical to each other, followed by heat treatment (heat setting) thereof. However, such a film has a disadvantage in that it is weak in strength, in particular, in the direction of thickness, and tends to break due to laminated stripping in a plane parallel to the film surface (which will hereinafter be referred to as cleavage). This disadvantage lowers the practical value of the PET film and, in some fields, restricts the use thereof.
In coating a PET film surface with a photographic emulsion layer, recording a magnetic layer or metallic plating layer, there often occurs breaking due to cleavage in the interior of the PET film itself, not in the interior of the coating layer or boundary thereof, resulting in deterioration of the value of the film, when force to strip the coating layer is applied to the coating composition. When such a PET film is used as a support of a photographic light-sensitive material for printing, it is impossible to carry out a cutting operation which is ordinarily required to partly take a printed image, because during the cutting operation cleavage takes place near the cut end, which cleavage causes irregular scattering of light and further trouble in the subsequent step of printing.
It is a principal object of this invention to provide a biaxially elongated PET film that has increased strength in the direction of thickness, and can be used without the formation of cleavage, whereby the above-mentioned disadvantages are overcome.
Previous attempts to overcome the foregoing disadvantages while retaining the excellent properties of PET film have been disclosed in Japanese Patent Publications Nos. 9859/69, 29274/69 and 31827/69. In Japanese Patent Publication No. 9859/69 there is mentioned, as a data recording material, a PET film which has an intrinsic viscosity of more than 0.82 and is biaxially elongated and heat set. The stripping resistance (cleavage resistance) of such a material is improved, but, as mentioned in this publication, the melt viscosity of PET having a high intrinsic viscosity is so large that it is very difficult to effect film making at machine commercial speeds. In chemcially synthesizing the raw PET material, it is well known that PET having a high intrinsic viscosity is economically disadvantageous.
In Japanese Patent Publication No. 29274/69, a method for improving the dimensional stability, strength in the direction of thickness and zero strength temperature is mentioned, which comprises copolymerizing one or more monomers capable of making a polymer photosensitive with a polyester such as PET, forming the polymer into a film and making the polymer three-dimension through bridging of the photosensitive unit. This method, however, has the disadvantage that a special copolymer is required as a raw material, and following the ordinary forming step a bridge making step by light radiation is necessary. Moreover, it is necessary to recover and reuse trimming edges which are formed during the production of a PET film in large amounts. This recovery is ordinarily carried out by crushing trimmed edges and then feeding them to the molding step in the form or polymer, or by chemically decomposing to the monomer and then reusing as a part of the raw material. The former is, of course, better. However, a film having three-dimensionally bridged bonds as mentioned in this publication cannot be re-melted and, therefore, there is no economical method for the recovery of trimmed edges.
In Japanese Patent Publication No. 31827/69 there is mentioned a method wherein 0.25 - 10% by weight of inactive titanium dioxide granules having an average size of 0.1 - 0.6 micron are incorporated into the film so as to lower the tendency of thin layer cleavage of a PET film used as a reinforcing film for textiles. The granules are added during the steps of forming the polymer into a film or synthesizing the polymer. Such addition needs a special technique, which may be complicated, to uniformly disperse the granules. Since high transparency is required for a PET film used as support of a photographic material, the lowering of transparency due to addition of such granules is such a serious disadvantage that it cannot be put to practical use.